1. Technical Field
This invention relates to an apparatus and method for precisely preloading a bearing onto a shaft. In one aspect, this invention relates to an apparatus and method for precisely preloading a spherical roller bearing onto a rotary die cutter shaft.
2. Background
Bearings are used in many different applications for supporting rotating equipment. Examples of bearings are ball bearings, needle bearings, tapered roller bearings, and spherical roller bearings.
Rotary die cutters cut many different kinds of materials. Examples of materials cut by rotary die cutters in large volumes are gaskets, labels, and stickers.
Spherical roller bearings contain an outer race, an inner race, and a plurality of freely rotatable spherical roller elements positioned between the two races. The inner race contains a tapered bore formed on the interior diameter of the inner race. The tapered bore mates with a taper machined into a shaft onto which the spherical roller bearing is to be mounted. Spherical roller bearings provide a high degree of runout precision for rotary die cutters. A pair of spherical roller bearings rotatably mount a rotary die cutter onto a shaft. The shaft mounted rotary die cutter then is assembled into a die cutting module.
When using spherical roller bearings, the bearings should be preloaded precisely to provide a precision mounting of the rotary die cutter and to extend the life of the bearing.
The tapered bore on the internal diameter of a spherical roller bearing should match up with a taper machined into the shaft onto which the bearing will be mounted. For operations using a rotary die cutter, a shaft will mount the rotary die cutter in a center position. The shaft will contain a pair of tapered surfaces, each located adjacent to one side of the rotary die cutter. A shoulder is machined into the shaft adjacent to each tapered surface. A thread is machined into the shaft adjacent to the tapered portions. A metal shim is slid onto the shaft and is positioned adjacent to the shoulder. The spherical roller bearings then are positioned onto the tapered surfaces. The thickness of each shim will dictate the distance an adjacent bearing will be able to move up the tapered surface. A lock nut then is threaded onto each end of the shaft. Each lock nut will contact the inner race of one of the bearings. As the inner race of each bearing expands, it causes the bearing to be preloaded onto the shaft. The lateral distance the bearing moves up the tapered surface then is determined by the thickness of the metal shim which has been slid onto The shaft.
If the bearing is not sufficiently preloaded, it will be sloppy, and the shaft to be supported will not revolve with the required precision. Conversely, if the bearing is excessively preloaded, it will fail prematurely requiring expensive downtime and replacement.
Different methods can be used to preload a bearing onto a shaft. One method uses a bearing gauge and gauge blocks to determine the proper shim thickness required to set the bearing preload. The bearing gauge and gauge blocks work on new bearings but cannot be used to preload reconditioned bearings because the size of the rolling elements will not be the same as those of new bearings. A second method involves the use of a string wrapped around the outer race of a bearing after it has been secured in place by the lock nut. The free end of the string is attached to a spring scale, similar to those used to weigh fish. As the string is pulled, the resistance of the roller elements of the bearing can be measured. This second method is subjective to human error and tends to be inaccurate, especially when performed by an inexperienced operator. Although the second method does work on both new and reconditioned bearings, it will not work when the bearings are packed with certain synthetic greases which do not have an extreme pressure additive.
Now, an apparatus and method have been developed for precisely preloading a new or reconditioned bearing onto a shaft. The apparatus and method also work on bearings which have been packed with certain synthetic greases not having an extreme pressure additive.
It is an object of the present invention to provide an apparatus and method for precisely preloading a bearing onto a shaft.
It is another object of the present invention to provide an apparatus and method for precisely preloading a spherical roller bearing onto a shaft.
Another object of the present invention is to provide an apparatus and method for precisely preloading a new or reconditioned bearing onto a shaft.
Still another object of the present invention is to provide an apparatus and method for precisely preloading a bearing onto a shaft even when the bearing has been packed with certain synthetic greases which do not have an extreme pressure additive. A further object of the present invention to provide an easy and reliable method for precisely preloading a spherical roller bearing onto a shaft.
Still further, an object of the present invention is to provide a relatively simple apparatus for precisely preloading a spherical roller bearing onto a shaft.
Still further, another object of the present invention is to provide a simple and straight forward method for precisely preloading a bearing onto a shaft which provides digital readouts of the measured torque in standard in-lb. units.
Other objects and advantages of the present invention will become more apparent to those skilled in the art in view of the following description and the accompanying drawings.
Briefly, the present invention provides an apparatus and method for precisely preloading a bearing onto a shaft, in one aspect, a spherical bearing. The apparatus includes a base plate and a rotary die shaft support mounted to and extending upward from the base plate. The baseplate has a swivel plate attached rotatably to the upper surface of the base plate, and the supports are attached to the swivel plate. The swivel plate swivels up to 360xc2x0 on the base plate and has a locking mechanism to retain the rotary die shaft in an axial relationship with a motor output shaft. The rotary die shaft has a shoulder, a tapered surface adjacent to the shoulder, and a threaded portion adjacent to the tapered surface. A shim is positioned on the shaft adjacent to the shoulder, and a bearing is positioned adjacent to the shim. The bearing has an inner race, an outer race, and a plurality of roller elements positioned between the inner race and outer race. The bearing is preloaded onto the shaft by a lock nut secured to the threaded portion of the shaft. The lock nut contacts the inner race of the bearing and forces the inner race to expand outwardly preloading the bearing onto the shaft. A motor mounted to the base plate provides the motor output shaft axially aligned with and connectable to the outer race of the bearing. The motor rotates the outer race of the bearing at a predetermined, constant speed. A force sensor attached to the motor measures the amount of torque required to rotate the outer race at the selected predetermined speed. The measured torque then is visually displayed on a display unit. The measured torque is controlled within a predetermined torque range to form the bearing precisely loaded.
The method of the present invention includes sliding a first shim onto the support shaft until it abuts the shoulder. The first shim has a predetermined thickness. A bearing then is slid onto the tapered surface, and a lock nut is tightened onto the threaded portion of the shaft. As the lock nut contacts the inner race of the bearing, the inner race expands, reducing the distance between the inner and outer races and preloading the elements of the bearing. The outer race of the bearing then is connected to a motor, and the motor is operated at a predetermined, constant speed. The torque required to rotate the outer race of the bearing is measured and compared to a predetermined torque range to provide a precisely preloaded bearing. In one aspect, the bearings are reconditioned bearings.